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by Matilda Thomas1, Jonathan D.A. Clarke1, Victor A. Gostin2, George E. Williams2 andMalcolm R. Walter3

The Flinders Ranges and surrounds, SouthAustralia: a window on astrobiology andplanetary geology1 Geoscience Australia, GPO Box 378, Canberra, ACT 2601, Australia. E-mail: matilda.thomas@ga.gov.au; jon.clarke@ga.gov.au2 Geology and Geophysics, University of Adelaide, SA 5005, Australia. E-mail: victor.gostin@adelaide.edu.au; george.williams@adelaide.edu.au3 Australian Centre for Astrobiology, University of New South Wales, NSW 2052, Australia. E-mail: malcolm.walter@unsw.edu.au

distinguished from simulation, for example the dunes of the StrzeleckiDesert are potentially good morphological and process analogues formartian dunes (Bishop, 1999) but they are not good simulations ofmartian dunes because of the different gravity, composition andatmospheric conditions between the Strzelecki Desert and Mars.

Australian geology and landscapes provide numerousopportunities for analogue research, in particular, Mars analogueresearch (West et al., 2009) because of the preservation of its longgeological history, including extensive hydrothermal activity and apredominantly arid climate. The diversity of processes and landformsfound in regions such as the Flinders Ranges and the surroundingarea in central South Australia, in particular the features found withinc. 100 km of Arkaroola, offer powerful insights into extraterrestrialand especially martian stratigraphy and geomorphology, along withastrobiology sites and examples. The Arkaroola region has been usedfor a range of Mars-related research and education activities includingremote sensing (Thomas and Walter, 2002; Brown et al., 2004),landscape evolution (Waclawik and Gostin, 2006), engineeringresearch related to planetary exploration (Waldie and Cutler, 2006)and education and outreach in all these disciplines (Laing et al., 2004;West et al., 2009).

This paper provides a brief description of some features in theFlinders Ranges and surrounds of interest to those studyingastronomical influences on terrestrial geology, Mars analogues, andastrobiology that will be visited during a field trip associated with the34th International Geological Congress (34 IGC). These are:

� Proterozoic and Phanerozoic rocks of the northern FlindersRanges

� Regolith and landscapes of the northern Flinders Ranges� Astrobiological features in the northern Flinders Ranges and

surrounds� Neoproterozoic stromatolites at Arkaroola� A possible Neoproterozoic deep hot biosphere� Fossil hydrothermal systems of the Mount Painter Inlier,

Arkaroola� Paralana Hot Springs, Arkaroola� Cryogenian glacials, Arkaroola� Late Cryogenian tidal rhythmites and Earth’s paleorotation, Pichi

Richi Pass� Ediacaran Acraman impact ejecta horizon, Bunyeroo Formation,

Central Flinders Ranges

The Flinders Ranges and its surroundings in SouthAustralia comprise an impressive rugged terrain thatrises abruptly from piedmont plains to the east and westand merges into the plains of the Cenozoic Lake EyreBasin to the north. Folded and faulted Neoproterozoic–Cambrian clastic and carbonate sedimentary rocks ofthe Adelaide Geosyncline (Adelaide Rift Complex) formthe predominant geology of the ranges and record varieddepositional environments and metamorphic overprintsand have had a complex landscape history, resulting ina diverse regolith. This ancient, arid terrain representssome of the best analogue landscapes and settings inAustralia to observe features and processes fundamentalto the evolution of the Earth. The strata of the FlindersRanges record the evolution of terrestrial surfaceenvironments and the biosphere through the Cryogenian,Ediacaran and Cambrian periods, including evidencefor Neoproterozoic glaciations, orbital and rotationaldynamics and asteroid impact. The diverse assemblagesof stromatolites, ancient and modern hydrothermalsystems, and alteration assemblages provide fieldlaboratories for astrobiological and hyperspectralresearch and training. For these reasons the northernFlinders Ranges near Arkaroola have been selected as asite for multi-disciplinary Mars analogue research andspace education.

Introduction

Analogue research involves an integrated set of disciplinesinvestigating terrestrial features and processes which can offer insightsinto understanding landscapes and processes observed on otherplanets. Analogue features also serve as training grounds for planetaryscientists and astrobiologists and as field test sites for equipment andinstruments used in planetary exploration. Analogues should be

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Geology of the northern Flinders RangesThe geology of the Neoproterozoic Adelaide Geosyncline (also

termed the Adelaide Rift Complex) is reviewed by Preiss (1987, 1993,2000), with Coats and Blissett (1971) focusing on the Mount Painterarea and its surrounds. The Paleozoic, Mesozoic and Cenozoicsuccessions of the region are reviewed by Drexel and Preiss (1995).These works remain unsurpassed in providing a framework tounderstanding the geology of the region.

Arkaroola is located in the northern Flinders Ranges (Figure 1),where rocks of the Mount Painter Inlier form a basement nucleusover which a younger Neoproterozoic succession was deposited. TheMount Painter Inlier comprises Mesoproterozoic metasediments andmetavolcanics (including the Radium Creek Metamorphics) intrudedby granites, pegmatites and minor amphibolite dykes. The highlyradiogenic nature of the Mesoproterozoic granites has resulted in along-lasting history of hydrothermal activity that has continued tothe present.

The Neoproterozoic succession includes numerous horizons ofstromatolitic carbonates, evidence for two Cryogenian glaciations,and an Ediacaran impact ejecta horizon derived from the Acramanimpact structure in the adjacent Gawler Ranges. The main phase of

deformation was the Ordovician Delamerian Orogeny at 514–490Ma (Drexel and Preiss, 1995; Foden et al., 2006). Ordovician granitesand pegmatites form a younger granite suite which intrudes theProterozoic basement (Hore et al., 2005).

Mesozoic and early Cenozoic sediments unconformably overliethe Proterozoic and Paleozoic rocks. This cover has been almostcompletely removed by later Cenozoic reactivation and uplift of thefolded strata to form the Flinders Ranges. The youngest Cenozoicsediments form piedmont alluvial aprons derived from the ranges.

Regolith and landscapes of the northernFlinders Ranges and surrounds

Regolith and associated paleolandscapes within the MesozoicEromanga Basin include a paleodrainage deposition (fluvial and minorglacial) in a landscape of moderate topographic relief and bedrockexposure (Davey and Hill, 2007; Hill and Hore, 2009). Cretaceousmarine transgressions across low-lying parts of the landscapedeposited chemically reduced clays and silts. The Mesozoic sedimentshave since been partially oxidised and tectonically disrupted,particularly during the Neogene (Davey and Hill, 2007). EarlyCenozoic regolith and landscape evolution is associated with thedevelopment of the Lake Eyre Basin region and Paleogene fluvialdeposits. Ephemeral fluvial, lacustrine, colluvial and aeoliandeposition, as well as tectonism and pedogenesis, have been spatiallyand temporally variable in the more recent geological evolution (Hill,2008).

The youngest Cenozoic landscapes of the plains to the east andnorth of the Arkaroola region record a complex evolution underchanging climatic conditions (Twidale and Bourne, 1996; Twidaleand Wopfner, 1996; Hill, 2008). Major alluvial fan systems occuralong the range front and drain into the adjacent salt lakes (Frome,Grace, Blanche and Callabonna) or into the dune fields on the marginof the Strzelecki Desert. The fans are variably duricrusted and dissectedand record a Quaternary history of fan deposition and incision relatedto both climate change and tectonics. Examples of these Cenozoicdeposits flank the Flinders Ranges at the Paralana Hot Springs (seebelow). The various surfaces, duricrusts and sediments provide ananalogue for the complex landforms to be expected on Mars. Someof these deposits, such as the mobile barchanoid sand dunes at GurraGurra Waterhole, have previously been studied as Mars analogues(Bishop, 1999).

Astrobiology in the Flinders RangesAstrobiology is the study of the origin, evolution, distribution,

and future of life in the universe. This multidisciplinary fieldencompasses the search for habitable environments... the search forevidence of prebiotic chemistry and life on Mars and other bodies inour Solar System, laboratory and field research into the origins andearly evolution of life on Earth, and studies of the potential for life toadapt to challenges on Earth and in space. From: “AboutAstrobiology”. (NASA Astrobiology Institute, 2008).

Neoproterozoic stromatolites at Arkaroola

There are numerous Neoproterozoic stromatolitic carbonates andassociated chert formations within the Adelaide Geosyncline, whichFigure 1 Regional geological context of the study area.

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may contain microfossils. These include the oldest carbonates of theCallanna Group and the Skillogalee Dolomite of the Burra Group,(both pre-Cryogenian), the Cryogenian interglacial carbonates of theBalcanoona, Etina and Trenzona formations, the Ediacaran WonokaFormation, and numerous horizons in the Cambrian (Coats, 1972;Preiss, 1987, 1993, and references therein). They attest to a flourishingphotosynthetic bacterial microbiota that was widespread in marine(and probably also lacustrine) environments ranging from peritidalto the base of the photic zone (c.100–150 m depth) during favourableconditions throughout the region. In the Arkaroola area, the bestpreserved stromatolites are found in the Balcanoona Formation aslarge reefs up to 1.1 km thick, with associated enigmatic structuresthat may be sponges (Giddings et al., 2009). Fore-reef, reef-marginand back-reef facies are all well represented.

The astrobiological significance of stromatolites lies in theirubiquity, as they are the oldest known macroscopic evidence of lifeon Earth and are found in diverse environments including saline andfreshwater lakes, intertidal flats, springs, hydrothermal vents, and bothshallow and deep marine. While not all stromatolite-like structuresare biogenic, not all biogenic stromatolites preserve microfossils orbiomarkers (Brasier et al., 2002; Schopf, 2006), and both microfossilsand biomarkers can be found in non-stromatolitic lithologies(Marshall, 2007). Recognition of stromatolite-like features on Marswould be a major discovery and would provide a focus for subsequentinvestigations. Therefore various researchers have suggested thatrecognition for stromatolitic morphologies be included in the searchfor life on Mars (e.g., Walter and Des Marais, 1993; McKay andStoker, 1989), as well as drawing parallels between terrestrialstromatolites and what might be found on Mars (e.g., Allwood et al.,2007; Van Kranendonk, 2006).

A possible Neoproterozoic deep hot biosphere

The Arkaroola region is also notable for the serendipitousdiscovery of possible evidence for a deep hot biosphere that inhabitedNeoproterozoic sedimentary successions during peak burial andmetamorphism. While examples of such organisms are known fromdeep wells today (Fyfe, 1996) and have been postulated to have livedas long as 3.8 billion years ago (Pinti et al., 2001), fossil examplesare poorly known. Structural investigations by Bons and Montenari(2005) examined fibrous anataxial calcite veins in the TindelpinaShale Member of the Tapley Hill Formation. These formed atc. 585 Ma at an estimated 3–6 km depth. Scanning electron microscopeobservations revealed about 1-micron-sized structures within theveins. Further studies showed that these structures were composed ofcalcite but contained higher sulfur than the surrounding material.Fluid inclusions in the calcite indicate a temperature of formation ofc. 60–80°C, and not exceeding 100°C. While a nonbiogenic origin ofthe objects is possible, it was considered unlikely (Bons et al., 2007).The weight of evidence from morphology, chemistry and sizedistribution indicates that the objects are fossilised microbes that livedin the veins at the time and depth of vein formation. Further work isneeded on the structures to test their biogenicity. If these features areindeed biogenic they are a potential analogue for a possible martianhabitat (Hoffmann and Farmer, 2000).

Mount Painter/Mount Gee

The Mount Painter area contains a complex collection of breccias,

which are characterised by large dyke-like bodies of siliceous hematiticbreccia and are interpreted to represent hydrothermal systems (Horeand Hill, 2009). This hydrothermal activity is most evident in thearea centred on Mount Painter and Mount Gee (Sprigg, 1945; Drexeland Major, 1987) with lesser expressions to the east at Livelys FindAu prospect (Collier, 2000) and to the northeast at the Hodgkinson Uprospect (Smith, 1992). Many of these breccias contain U and minorsulfide mineralisation. The breccias occur as irregular bodies withinthe basement complex, adjacent to a zone of extensive faulting, whichalso contains Ordovician granites and pegmatites (Lambert et al.,1982). The Mount Gee system includes a range of features includingcrustiform and colloform textures, bladed and replacement carbonate,fluorite, zeolites and siliceous fluids resulting in cavity-fillcrystallisation and jasperoidal formations as seen in Figure 2 (Hore,2008). The jasperoidal unit at Mount Gee could have been depositedas a gel (e.g., Eugster and Jones, 1967), and many textures in theMount Painter and Mount Gee silica deposits are similar to thebotryoidal silica surfaces observed at the Sleeper Au deposit in Nevada(Saunders, 1994) and the recrystallised silica gels from YellowstoneNational Park (Fournier et al., 1991). While the preservation ofmicrofossils would appear likely in the Mount Painter hydrothermaldeposits, preliminary attempts to find them have been unsuccessful(Carlton, 2002).

Figure 2 Mount Gee epithermal quartz.

Ancient hot-spring deposits and associated hydrothermalalteration exemplify key environments in the exploration and studyof earliest life on Earth (e.g., Bock and Goode, 1996). These systemscould have similar significance in the search for extraterrestrial life,and particularly on our nearest neighbour, Mars, where hydrothermalactivity is likely to have occurred in the past and may even continuesomewhere underground today (e.g., Walter and Des Marais, 1993;Catling and Moore, 2003). Martian hematite-precipitating springdeposits are high-value targets for martian astrobiology missions, inparticular sample return missions (Catling and Moore, 2003; Allen etal., 2001, 2004). The hematite-rich shallow hydrothermal systems ofthe Mount Painter Inlier are therefore of considerable astrobiologicalinterest as Mars analogues (Thomas and Walter, 2002, 2004; Bruggeret al., 2011) because of the high preservation potential for bothmicrofossils and organic matter in the silica-hematite precipitates.The high radiation environment of these systems, of which ParalanaHot Springs (see below) is the current example, provides another

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similarity to the surface of Mars, while the association of hydrothermalactivity with an icy surface environment in the Mount Painterhydrothermal system appears unique for a fossil terrestrialhydrothermal system (Brugger et al., 2011) and is a further parallelfor what might be found on the surface of Mars.

Alteration (including hydrothermal) can be detected inhyperspectral data in regions like the Mount Painter Inlier (Thomasand Walter, 2002; Brown and Thomas, 2004; Brown et al., 2004).Studies indicate that careful band selection and testing in terrestrialfield analogues are required if spacecraft systems are to detecthydrothermal alteration on Mars and differentiate it from more regionalalteration signatures.

Radioactive hot springs of the Paralana FaultZone

The Paralana Fault Zone, consisting of northeast-trending faults,runs along the eastern margin of the Mount Painter Inlier, northeastof Arkaroola (Figure 1). The fault zone is associated with an area ofanomalously high heat flow attributed to high concentrations ofradioactive elements in Mesoproterozoic granites of the adjacentMount Painter Inlier and is generally believed to be the main conduitfor hydrothermal fluid dispersal in the area (Coats and Blissett, 1971;Thomas and Walter, 2002; Brugger et al., 2005). Hydrothermal activityduring the Paleozoic has produced large volumes of uraniferousbreccias, siliceous deposits (possibly sinters), and Cu-Fe and Fe-Udeposits. Leaching of these deposits may have contributed to thesecondary U deposits in Cenozoic sandstones east of the Inlier(Brugger et al., 2005). The Paralana Hot Springs occur to the northeast of Mount Painter on the Paralana Fault Zone (Figures 1 and 3).The springs are evidence of continuing hydrothermal activity alongthe fault zone. The source of this fluid and of past hydrothermal fluidsis difficult to ascertain, although it is most likely to be predominantlymeteoric in origin (Foster et al., 1994) representing the surfaceexpression of a cyclic, low-temperature, non-volcanic hydrothermalsystem (Brugger et al., 2005).

Water from the Paralana Hot Springs is neutral (pH 7–8) and thesprings discharge 16 L/s at a temperature of 57°C. The 5 ppm F,33 ppb Mo, 11 ppb W, 16 ppb Cs and 200 ppb Rb concentrations inthe spring water are comparatively high. δ13C values of CO2 (g)

emanating from the springs and dissolved HCO3– suggest that the

carbon source is organic matter, e.g., soil or plants. Radonconcentrations at the springs are very high (radiation of 10,952 Bq/m3), suggesting a localised radiogenic source at shallow depth(Brugger et al., 2005). These radiogenic springs are not associatedwith any active volcanism, and therefore provide an opportunity tostudy the effects of amagmatic heat sources, which could drive ahydrothermal system for an extended timeframe, perhaps a billionyears or more (Brugger et al., 2011).

Studies of hot springs have shown they support a high diversityof bacteria and Archaea. The hot spring environs at Paralana havebeen studied as an analogue for early life, and microbial adaptationto extreme temperatures and radioactivity levels (e.g., Anitori et al.,2002). The composition of microbial ecosystems around subaerialhot springs and submarine vents is significant to the understandingof early life on Earth and tracing evolutionary phylogenic trees, andinterpreting presumed biomarkers (microfossils, lipids, isotopes etc.)found in these ancient systems (Walter and Des Marais, 1993).Analysis of bacterial communities by Anitori et al. (2002) indicatedthat a diverse community of bacteria is supported by the waters at theParalana Hot Springs. The presence of Ra gas, which bubbles upintermittently in the thermal source pool, makes the Paralana HotSprings highly suited as an analogue for ionising radiationenvironments, which may have been common on the early Earth andMars.

Cryogenian glacialsSedimentological and paleomagnetic studies of Cryogenian glacial

deposits in South Australia have been a stimulus for worldwidemultidisciplinary research on the nature and extent of Cryogenianglaciations. Two major glaciations are recorded in the succession,the earlier Sturt glaciation (Preiss et al., 2011) and the later Elatinaglaciation (Lemon and Gostin, 1990; Williams et al., 2008, 2011),often incorrectly referred to as the Marinoan glaciation.

The late Cryogenian (c. 630 Ma) Elatina glaciation in SouthAustralia is represented by a spectrum of facies (Figure 4). Theperiglacial–aeolian Whyalla Sandstone on the Stuart Shelf passeseastwards into glaciofluvial, deltaic, littoral and inner marine-shelfdeposits of the Elatina Formation and outer marine-shelf diamictiteand mudstone-siltstone with dropstones in the Adelaide Geosyncline.Important features of the Elatina glaciation include the widespreadand persistent rainout of fine-grained sediment and ice-rafted debris,grounded ice on diapiric islands within the basin and on cratonicregions in the east (but absent on the Gawler Craton in the west).Other key features include permafrost near sea level with a stronglyseasonal periglacial climate, glaciofluvial deposition, several glacialadvances and retreats, wave-generated ripple marks, and the annual(seasonal) oscillation of sea level (Preiss, 1987; Lemon and Gostin,1990; Williams et al., 2008, 2011). The plaque for the GlobalStratotype Section and Point (GSSP) for the recently establishedEdiacaran System and Period (Knoll et al., 2006) is placed betweenthe Elatina Formation and the overlying Nuccaleena Formation inEnorama Creek in the central Flinders Ranges.

The late Cryogenian glacials in South Australia were depositedat low paleolatitudes (Figure 4, inset), as shown by high-qualitypaleomagnetic data for red beds from the Elatina Formation thatdemonstrate the early acquisition of magnetic remanence anddeposition within 10° of the paleoequator (Schmidt et al., 1991, 2009;

Figure 3 Paralana Hot Springs source pool and microbialcommunities.

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Schmidt and Williams, 1995; Sohl et al., 1999). However, the indicatedpaleogeography, with extensive and long-lived open seas, unglaciatedcontinental regions and an active hydrological cycle, conflicts withthe “snowball Earth” hypothesis of Hoffman and Schrag (2002).

The older, Sturt glaciation is equally well represented by diversefacies throughout the Flinders Ranges (Preiss, 1987, 1993; Youngand Gostin, 1989). At Stubbs Waterhole, 5 km from Arkaroola,spectacular thick-bedded sandy boulder and pebble conglomerateswith diverse lithologies were deposited from vigorous glacialmeltwater streams. It is believed that during the middle Cryogenianan elevated large ice cap covered the whole area between the MountPainter Inlier and Broken Hill (250 km to the southeast). Majorthickness variations of the c. 500 m thick Bolla Bollana Formationreflect rapid subsidence in rifted basins.

Tidal rhythmites and Earth’spaleorotation, Pichi Richi Pass

The Elatina Formation near the margins of the AdelaideGeosyncline includes tidal rhythmites of siltstone and fine-grainedsandstone deposited on a series of ebb-tidal deltas and estuarine tidalflats that formed during a high stand of sea level during temporary

glacial retreat (Williams, 1991, 2000; Williams et al., 2008). The fine-grained sediment load of ebb tidal currents is related directly to tidalrange (or maximum tidal height), and deposition offshore from ebb-tidal jets and plumes forms neap–spring cycles comprising semidiurnaland diurnal (lunar day) graded laminae mostly of fine sand and silt,with mud bands deposited during slack water at neaps (Figure 5).The rhythmite unit is 18 m thick at Warren Gorge (Williams, 1996)and somewhat thinner at a more distal setting in Pichi Richi Pass(Figure 4) where exposure is limited.

Detailed study of cores from three vertical holes drilled throughthe rhythmite unit in Pichi Richi Pass have provided an internally-consistent paleotidal data-set comprising numerous tidal cyclesranging from semidiurnal to the lunar nodal cycle (9.4 m log of 1580successive fortnightly neap–spring cycles recording 60 years ofcontinuous deposition; Williams, 1991). The neap–spring cyclescontain 8–16 diurnal laminae, with many cycles abbreviated at neaps(Figure 5b); semidiurnal increments occur locally, and are conspicuousin thicker neap–spring cycles from tidal rhythmites in the correlativeReynella Siltstone Member near Hallett Cove, 300 km to the south(Figures 5a and 6). Paleotidal cycles resulting from variation in thethickness of successive neap–spring cycles compare closely withmodern tidal patterns for Townsville, Queensland (Figure 7): featurescommon to both data sets include first-order peaks marking the solaryear and the annual oscillation of sea level, second-order peaksmarking the semiannual tidal cycle, and a sawtooth pattern reflectingalternate high and low spring tides due to the eccentric lunar orbit.The duration of Elatina rhythmite deposition matches the c.70 year

Figure 4 Facies of the late Cryogenian Elatina glaciation. Isopachsin metres. CFZ = Central Flinders Zone, NRZ =North FlindersZone. 1, 2, 3 = diapiric islands. Paleowind rose diagram for theWhyalla Sandstone. Inset shows paleolatitudes and paleowinddirection (arrow). After Williams et al. (2008).

Figure 5 Neap–spring cycles of late Cryogenian tidal rhythmites.Scale bar 1 cm, N = neap-tidal muddy bands. (a) Reynella SiltstoneMember; diurnal laminae have mudstone tops (arrows) andcomprise semidiurnal increments. (b) Elatina Formation, PichiRichi Pass. From Williams (2004).

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morphologic cycle for a modern ebb-tidal delta (Ping, 1989).Comparable tidal rhythmites occur in modern glaciomarine settings(Smith et al., 1990; Cowan et al., 1999).

Information on the Earth’s paleorotation and the Moon’s orbit inlate Cryogenian times revealed by time-series analysis of the Elatinacore log, supplemented by data for the Reynella Siltstone Member,includes 13.1 ± 0.1 lunar months/year, 400 ± 7 solar days/year,21.9 ± 0.4 hours/solar day, and a mean Earth–Moon distance of

96.5 ± 0.5% of the present distance(Williams, 1991, 2000). The rhythmitesalso record the non-tidal, annualoscillation of sea level (Figure 7), whichis a response mostly to seasonal changesin water temperature as well as variationin winds and atmospheric pressure,indicating extensive open seas during lateCryogenian glaciation (Williams et al.,2008, 2011).

Acraman impact ejectahorizon in BunyerooGorge

The Acraman impact ejecta horizon(AIEH) in the mid-Ediacaran (c.580 Ma)

Figure 6 Stratigraphy of the late Cryogenian Yerelina Subgroup, which encompasses all the unitsof the Elatina glaciation in South Australia. After Preiss et al. (1998) and Williams et al. (2008),with Marinoan sequence sets M2.1–M2.3 and relative sea-level curves from Preiss (2000).

Figure 7 Tidal patterns for (a–c) the late Cryogenian Elatina rhythmites and (d–f) Townsville, Queensland, for 19 October 1968 to 3 June1970. (a, d) Unsmoothed curves. (b, e) Smoothed curves. (c, f) Residuals (a minus b; d minus e), showing 180° phase changes in thesawtooth patterns. From Williams (1991, 2000).

Bunyeroo Formation (Gostin et al., 1986, 1989) was the firstPrecambrian impact ejecta deposit to be linked to a specific astrobleme,the Acraman impact structure on the Gawler Craton, several hundredkilometres to the west (Williams, 1986). The AIEH consists of asolitary thin layer of red dacitic fragments and sand, enveloped by areduction halo of grey-green shale, and is well exposed within marine-shelf maroon shale 80 m above the base of the 400 m thick formation(Figure 8). The volcanic fragments show features of shock

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metamorphism and the horizon is anomalous in cosmogenicsiderophile elements, including Ir (Gostin et al., 1989). U-Pb zircondating, paleomagnetic data and the regional distribution of the ejectaconfirm derivation of the volcanic fragments from the Acraman impactstructure in the 1592 Ma Yardea Dacite on the Gawler Craton(Compston et al., 1987; Schmidt and Williams, 1996; Williams andWallace, 2003; Williams and Gostin 2005). Typical sections of theejecta are shown in Figure 9 and ejecta regional distribution in Figure10.

The magnitude, age and potential environmental effects of theAcraman impact and the nature of the AIEH were reviewed byWilliams and Wallace (2003) and Williams and Gostin (2005). TheAcraman impact structure is complex, with a transient cavity 40 kmin diameter and a final structural rim (90 km diameter) that is eroded≥2.5 km below the original crater floor. The AIEH in the AdelaideGeosyncline occurs at radial unfolded distances of up to 400 km fromthe centre of the Acraman structure. The impact occurred at a low (c.12.5°) paleolatitude and probably perturbed the atmosphere in boththe northern and southern hemispheres. The dimensions of Acraman

Figure 8 Stratigraphic column for Ediacaran strata in the centralAdelaide Geosyncline. AIEH = Acraman impact ejecta horizon. Thefamous metazoan fossil assemblage (Ediacara biota) occurs nearthe base of the Rawnsley Quartzite (Preiss, 1987, 2000). AfterWilliams and Gostin (2005).

Figure 9 Principal styles for the ejecta horizon in the BunyerooFormation. Style 1 (Bunyeroo Gorge–Brachina Gorge area)represents primary fallout deposited from suspension. Style 2(western Adelaide Geosyncline) represents ejecta reworked byimpact-induced tsunamis. After Wallace et al. (1996).

Figure 10 Location of the Acraman structure and the Acramanimpact ejecta horizon in mid-Ediacaran strata. Solid circles indicateejecta seen in outcrop, and open circles ejecta recorded in drill core.Triangles indicate ice-rafted detritus and ejecta seen in closestratigraphic proximity. From Gostin et al. (2010).

structure indicate that the impact would have caused earthquakes,tsunamis, atmospheric ozone loss, and the global dispersal of adust cloud that lowered light levels below those required forphotosynthesis.

The presence of ice-rafted dropstones, granule clusters, frozenaggregates and till pellets in mudstones of the lower BunyerooFormation in the central Adelaide Geosyncline (Figures 8 and 10)and in the correlative Dey Dey Mudstone (Officer Basin) implies acold climate and nearby areas of freezing and glaciation both beforeand after the Acraman impact (Gostin et al., 2010). Following

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glaciation and impact, the overlying Ediacaran succession shows anegative carbon isotopic excursion, rapid diversification ofacanthomorph acritarchs, and biomarker hydrocarbon anomalies,climaxing with the advent of metazoans. Release from the combinedenvironmental stresses of glaciation and impact may have expeditedbiotic evolution in the later Ediacaran (Grey et al., 2003; McKirdy etal., 2006; Gostin et al., 2010).

ConclusionsThis paper highlights a selection of features in the central and

northern Flinders Ranges that illustrate important events and processesin the physical evolution of the Earth as a planet, including thewidespread Neoproterozoic glaciations, the rotational and orbitalevolution of the Earth-Moon system, and asteroid impact.Stromatolitic successions provide examples of their sedimentary roleand morphological evolution during the Cryogenian through to theCambrian. Also highlighted are some of the features that provideanalogues for possible sites on Mars that would be prime targets inthe search for extraterrestrial life. These features exemplify theconsiderable scope for study and understanding of astrobiology andplanetary science that exists in the Flinders Ranges and surrounds.

AcknowledgementsWe thank Phil Schmidt, Narelle Neumann, John Keeling and

Natalie Kositcin for their helpful comments and reviews. MatildaThomas and Jonathan Clarke publish with the permission of the ChiefExecutive Officer of Geoscience Australia.

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Matilda Thomas is a geologist atGeoscience Australia and has workedin regolith mapping, mineral explo-ration and applied spectroscopy since2002. She also worked as a researcherfor the space science exhibition “ToMars and Beyond” at the NationalMuseum of Australia. Her interestsinclude hyperspectral technologies andplanetary science, as well as speleologyand extremophile ecology.

Jon Clarke is a geologist with 25 years’experience, mostly in the minerals andenergy industries and has also workedin the university and governmentsectors. He currently works in the fieldsof aquifer mapping and characteri-sation across Australia for GeoscienceAustralia and is vice president of MarsSociety Australia. Jon’s Mars interestsinclude terrestrial analogues of martianfeatures including springs, slopes, andinverted channels, habitat design.

George Williams received an MSc ingeology from the University of Mel-bourne in 1963 and a PhD in sedi-mentology from the University ofReading in 1966. His subsequent workhas been divided between industry andresearch based at the University ofAdelaide, much in association with theCSIRO. He was awarded a DSc by theUniversity of Reading in 1998.

Victor Gostin is a retired AssociateProfessor in Geology and Geophysicsat the University of Adelaide. Hisscientific interests include the originsand evolution of the solar system andof life, meteorite impacts, earth history,environmental geoscience, and theeffects of natural phenomena on thecourse of human history. His otherinterests include sketching theAustralian outback.

Malcolm Walter is a Professor ofAstrobiology at the University of NewSouth Wales in Sydney. He is Directorof the Australian Centre for Astro-biology based at that university. He hasworked for 35 years on the geologicalevidence of early life on Earth,including the earliest convincingevidence of life. He is a member of theExecutive Council of NASA’s Astro-biology Institute. He also works as anoil exploration consultant.